This invention relates generally to optical fibers and particularly to optical fibers used in forming fiber optic rotation sensors. Still more particularly, this invention relates to an optical fiber that absorbs residual pump light introduced into a fiber optic rotation sensor from an optically pumped broadband fiber light source.
A fiber optic ring interferometer typically comprises a loop of fiber optic material that guides counter-propagating light waves. The light waves typically originate in a solid state optical source such as a superluminescent diode. Rotation of the loop creates a relative phase difference between the counter-propagating waves by the well known Sagnac effect. The amount of phase difference is a function of the angular velocity of the loop. After traversing the loop, the counter-propagating waves are combined so that they constructively or destructively interfere to form an optical output signal.
The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing it to determine the rotation rate.
Superluminescent diodes provide coherent light having the desired broad bandwidth. However, superluminescent diodes are expensive and have operating lifetimes that are too short for use in many navigation systems. A triaxial gyro for measuring rotations about three mutually perpendicular axes as required for navigation may be formed using three SLD light sources. A triaxial gyro may also be formed with a plurality of optical couplers arranged to divide the light output from a single SLD source to provide three separate beams. Both of these approaches for providing suitable light beams for a triaxial gyro are expensive and do not solve the short lifetime problem associated with SLDs.
A laser diode pumped rare earth doped fiber source produces light having the desired broad bandwidth, long operating lifetime and wavelength stability. Part of the pump light is absorbed by the gain medium and part may be reflected. However, the optical output an optically pumped source typically includes some of the pump light. The pump light that is in the output is known as residual pump light (RPL). About 10% to 30% of the pump power is output as (RPL) from a broadband fiber source.
RPL degrades the performance of a fiber optic rotation sensor by increasing the noise and instability. No simple solution has been found in literature to prevent the RPL from being introduced into a fiber optic rotation sensor from an optically pumped source.